This invention relates generally to a light diffusing article in the form of film or multi-wall sheet and manufacturing process thereof.
It is known in the prior art to improve the light diffusion properties of thermoplastic substrates such as polyester or polycarbonate substrate by the addition of incompatible inorganic minerals, e.g., BaSO4, a commonly used white pigment. Besides BaSO4, other minerals that may be used are aluminum oxide, zinc oxide (ZnO), calcium sulfate, barium sulfate, calcium carbonate (e.g., chalk), magnesium carbonate, sodium silicate, aluminum silicate, silicon dioxide (SiO2, i.e., silica), mica, clay, talc, and the like in a range of up to about 25 weight percent. Thicker substrates require less inorganic minerals to attain the same diffusion properties. The action of these minerals is to cause formation of cavities or voids in the substrate. These cavities contribute to making the substrate more opaque due to multiple light scattering. The concentration of the minerals and their particle sizes determine the void structure and several substrate properties, i.e., the higher the concentration of the minerals, the lower the molecular weight of the resulting thermoplastic resins, resulting in impaired impact strength and other physical and mechanical properties.
EP 0 604 130 B1 discloses polycarbonate compositions comprising about 0.001 to 10 wt. parts poly(methyl silsesquioxanes) having good light dispersion properties despite the high rate of light transmission. The composition can be advantageous in applications requiring high light transmission and high integrity for safety, such as sky lights, walls, and other large-scale lighting covers such as signs.
JP 03 207743 discloses articles such as lightening units, illuminated signs and displays having high light transmittance, which compositions comprise methacrylate resin and about 0.1 to 10 wt. % of poly(methyl silsesquioxanes) with a particle size of about 0.5 to 20 microns.
JP unexamined patent publication no. 2001-66701 discloses light-transmitting thermoplastic composition comprising scattered particles of about 1 to 8 xcexcm. in materials such as polycarbonate and methacrylate resins, for applications such as lighting fixtures, sign boards, glazing and screens for projection television. In one embodiment, the scattered particles are coated with a polyorgano silsesquioxane. In another embodiment, the composition is used in a light transmission screen for use in liquid crystal display (LCD) applications.
U.S. Pat. No. 5,372,879 discloses the use of silicone resins, e.g., Tospearl 120, in a polyester composition such as polyethylene terephthalate for magnetic recording base, for a film with a surface having a small number of large protuberances, excellent running property and excellent winding property. In one embodiment, the surface roughness of the polyester film is 0.022 xcexcm as measured using JIS B0601.
The specifications applied to plastic sheets or films (xe2x80x9csubstrates) in a number of multi-wall sheet applications and optical applications in general, are rather stringent, including the requirements that the substrates should not form any bubbles or cavities when thermoplastically processed; they should display minimal optical birefringence; they should have a low thickness tolerance or variation, low curvature, low thermal shrinkage, and low surface roughness. Backlit flat panel displays (LCD) utilize a cold cathode florescent lamp as a light source. This cylindrical source is directionally reflected towards an injection molded wedge more commonly referred to as a light guide. The light guide is utilized to help provide an evenly distributed source of light from the backlight to the viewer. The light that results from the light guide requires further distribution to provide the uniform intensity that is required. This is commonly accomplished with a film with light diffusion type functionality to provide light spreading and decorelation type functions.
Applicants have surprisingly found that the surface quality of light diffusing articles in the form of multi-wall sheets and films (xe2x80x9csubstratexe2x80x9d) can be significantly improved by the addition to a light transmitting resin composition comprising the substrate such as polycarbonate, a sufficient amount of a polyalkyl silsesquioxane or a mixture thereof, whereby the alkyl groups can be methyl, C2-C18 alkyl, hydride, phenyl, vinyl, or cyclohexyl.
The invention relates to methods to improve surface quality of a light diffusing article by adding to the thermoplastic composition comprising the article a sufficient amount of a sufficient amount of a polyalkyl silsesquioxane or a mixture thereof, whereby the alkyl groups can be methyl, C2-C18 alkyl, hydride, phenyl, vinyl, or cyclohexyl.
The invention further relates to a light diffusing article comprising 0.001 to 10 wt. parts per 100 wt. part of a light transmitting plastic resin, of a polyalkyl silsesquioxane or a mixture thereof, whereby the alkyl groups can be methyl, C2-C18 alkyl, hydride, phenyl, vinyl, or cyclohexyl.
In one embodiment of the invention, the light diffusion article is characterized by having a surface roughness of: a) a center line average roughness Ra of 0.1 xcexcm or less, b) a ten-point average roughness Rz of 0.5 xcexcm or less, and c) a maximum height surface roughness of 1 xcexcm or less.